1. Field of the Invention
The present invention relates to a circuit for a stepping motor and, more particularly, to the improvement of a driving circuit for a stepping motor which enables a smooth seeking by inserting an inner step pulse between the step pulses which are input so as to make the seeking interval smaller than the interval between the input step pulses.
2. Description of the Related Art
Stepping motors which are capable of securing a predetermined amount of rotation and a predetermined rotational position in correspondence with the number of step pulses input are used in various industrial fields to obtain the accurate seeking position. Stepping motors are advantageous in that the accurate positional control is enabled with a simple structure.
Stepping motors are very suitable as carriage driving motors for seeking a reading/writing head of a floppy disk drive, a hard disk drive or the like to a predetermined track.
Various driving systems having various combinations of the number of exciting coils and the number of exciting phases are adopted for such stepping motors. A given driving system is selected from these systems in accordance with the characteristics of the object of control.
FIG. 6 shows a conventional stepping motor having a rotor 10 which carries the main shaft of the motor, and fixed exciting coils 12, 14 having a phase A and a phase B, respectively, and provided around the rotor 10. The main shaft of the motor is driven in steps with a high accuracy by exiting the coils 12, 14 by a predetermined combination of a forward current and a reverse current.
FIG. 7 shows an example of step pulses for driving the stepping motor shown in FIG. 6. In an ordinary case, the stepping seeking is realized by exciting the exciting coils 12, 14 by a different combination of a forward current and a reverse current every time a step pulse is input.
In FIG. 7, when a step pulse SP.sub.1 is input, both the exciting coils 12, 14 are excited in the forward direction, in other words, in the phases A and B, and the rotor 10 is positioned at the position indicated by the arrow AB in FIG. 6. When the next step pulse SP.sub.2 is input, the exciting coil 12 is excited in the reverse direction while the exciting coil 14 is excited in the forward direction, namely, in the phases AB, and the rotor 10 is rotated in this state as far as the position indicated by AB in FIG. 6. Similarly, when the step pulses SP.sub.3 and SP.sub.4 are subsequently input, the rotor 10 is positioned at the position AB and AB, respectively, by two-phase excitation. In this way, the four seeking positions are obtained during one revolution of the rotor 10 in FIG. 6.
However, in such two-phase excitation, the rotor 10 is subjected to acceleration and deceleration in repetition at every switching of the currents. As a result, the rotational speed of the rotor 10 greatly varies, so that, for example, when the head carried by the carriage of a floppy disk drive is moved to a comparatively distant track, a large speed variation is generated, which leads to the problem such as the loss of energy, generation of a noise and the increase in wear.
In order to eliminate such non-uniform rotation and to obtain a smooth rotor rotation, a method of obtaining a smooth rotation by inserting an inner step pulse at an intermediate position between the input step pulses in the driving circuit of the step motor when the step pulse is received has conventionally been put to practical use.
As shown in FIG. 7, such an inner step pulse is output substantially at the central portion of the interval of step pulses. Therefore, the inner step pulse is formed by adding a predetermined delay time .tau. to the pulse interval T of the step pulse SP, and a composite pulse train is produced by compounding the step pulse SP and the inner step pulse. On the basis of this composite pulse train, the exciting coils 12, 14 are excited.
Consequently, in FIGS. 6 and 7, after the excitation in the phases A and B at the step pulse SP.sub.1, the exciting coil 14 alone is excited in the forward direction, in other words, B-phase excitation is carried out when the inner pulse is received, whereby the rotor 10 is rotated as far as the position B. Similarly, the subsequent inner pulses are output, and the rotor 10 is rotated to the positions A, Band A, respectively.
According to the two-phase/one-phase excitation including such inner pulses, it is possible to obtain a very smooth rotational operation of the rotor 10.
However, since the delay time of the inner step pulse in the related art is fixed at, for example, .tau..sub.0 in FIG. 7, when the pulse interval T of the input step pulse SP is constant, the inner step pulse effectively acts, but when the pulse interval T varies, a large nonuniformity is rather produced on the rotation of the rotor 10.
In FIG. 7, the interval of the step pulse T.sub.1 is set at 3 ms. In this case, the stepping motor can be used for seeking the carriage of a floppy disk drive, for example. In the floppy disk drive, the step pulse greatly varies in accordance with the command from the host computer and the type of the program. For example, in FIG. 7, step pulses output at intervals T.sub.2 =5 ms and T.sub.1 =3 ms are supplied and step pulses are input to the same floppy disk drive at different intervals T.
Therefore, in the above-described driving system in which the delay time .tau..sub.0 is fixed at 1.5 ms, when step pulses are output at an interval T.sub.2 of 5 ms, very irregular drive pulses are provided, which rather produces a large nonuniformity on the rotation of the rotor 10.